Total body K content depends on the balance between intake and output, the latter regulated primarily by renal K secretion in the connecting (CNT) and collecting ducts. The magnitude of K secretion in these segments is determined by the electrochemical gradient and the permeability of the apical membrane to K. Two apical K-selective channels have been identified in the CNT and cortical collecting duct (CCD): a low- conductance SK and a high-conductance Ca/stretch-activated BK (or maxi-K) channel. Whereas the SK channel is restricted to Na and K transporting principal cells, the density of BK channels is highest in acid- base transporting intercalated cells. Recent published and preliminary studies by our lab have provided compelling evidence that the SK channel mediates baseline K secretion and that the BK channel participates in flow-stimulated Ca-dependent K secretion. Furthermore, we have reported that the rates of flow-stimulated K secretion and levels of BK channel expression in the CCD are regulated during postnatal development and in response to chronic changes in dietary K intake. Based on this data, we hypothesize that flow-stimulated net K secretion in the distal nephron is mediated by BK channels in intercalated cells in a Ca-dependent manner, and that the regulation of channel activity during development and in response to epigenetic factors is determined by the differential expression of BK alpha subunit splice variants and beta subunits. This hypothesis will be tested using an integrated approach including functional (in vitro microperfusion, fluorescence functional imaging, electrophysiology), biochemical, and molecular approaches, generally applied to mammalian CCDs. In SA1, we will define the relationship between the transient flow-induced increase in cell Ca concentration and sustained stimulation of K secretion, focusing on the, timing and source of the Ca leading to the response, and the role of Ca/calmodulin kinase in the signal transduction pathway. In SA2, we will examine the molecular regulation of channel variant and isoform expression during normal postnatal development, and in response to acute changes in dietary K intake and metabolic acid-base disturbances;these studies will be interpreted in the context of observed effects on flow-stimulated K secretion. The results of this investigation should provide new insight into the molecular physiology underlying renal K adaptation in health and disease (e.g., Bartters syndrome), and extend our emerging understanding of the biomechanical regulation of epithelial cell function. Lay summary: We have recently identified a "BK" channel (pore) in kidney tubules that is activated at high urinary flow rates, which subject the cells to shear, stretch and an increased cell calcium concentration. The goal of this application is to enhance our understanding about the expression, ontogeny, localization, and regulation of BK channels in the mammalian kidney under conditions of health and disease.